1. Field of the Invention
The present invention relates to a webbing retractor which structures a seat belt device of a vehicle or the like.
2. Description of the Related Art
A seat belt device which restrains, by an elongated, strip-shaped webbing belt, the body of a vehicle occupant who is seated in a seat of a vehicle, is equipped with a webbing retractor which is fixed to the vehicle body at the side of the seat. The webbing retractor has a spool whose axial direction runs, for example, substantially along the longitudinal direction of the vehicle. The proximal end side, in the longitudinal direction, of the webbing belt is anchored at the spool. The webbing belt is taken-up in layers around the outer peripheral portion of the spool.
An urging member, such as a torsion coil spring or the like, which urges the spool in a take-up direction in which the spool takes up the webbing belt, is provided at the webbing retractor. Due to the urging force of this urging member, the webbing belt is taken up and accommodated. Further, in the state in which the webbing belt is applied to the body of a vehicle occupant, slack or the like of the webbing belt is eliminated by the urging force of the urging member. In this type of webbing retractor, the webbing belt, which is wound on the spool, can be pulled out by the vehicle occupant pulling, against the urging force of the urging member, a tongue plate provided at the longitudinal direction intermediate portion of the webbing belt. The tongue plate is made to be held at a buckle device provided at the side of the seat which is opposite the side at which the webbing retractor is disposed. The webbing belt is thereby applied to the body of the vehicle occupant.
Usually, a pretensioner and a lock mechanism are provided in order to strongly restrain the body of the vehicle occupant when the vehicle rapidly decelerates or the like.
The pretensioner has a cylinder at whose interior a piston is accommodated so as to be freely slidable. A rack bar is provided at the piston. When the vehicle rapidly decelerates, a gas generating agent combusts, and gas is generated suddenly. In this way, the internal pressure of the cylinder rises, the piston slides, the rack bar meshes with a pinion which is mechanically connected to the spool, and the pinion rotates in the take-up direction.
A clutch mechanism is provided between the pinion and the spool. The clutch mechanism does not transmit the rotation of the spool to the pinion, but transmits the rotation of the pinion to the spool. When the pinion rotates in the take-up direction, the spool also rotates in the take-up direction.
At the time when the vehicle rapidly decelerates, the pretensioner rotates the spool in the take-up direction, such that the wiebbing belt is taken-up onto the spool and the force by which the webbing belt restrains the body of the vehicle occupant increases.
The lock mechanism has a base lock which rotates together with the spool, and an inertial plate which can rotate relative to the spool. A spring is interposed between the base lock and the inertial plate. When the base lock rotates together with the spool, the spring, which rotates together with the base lock, pushes the inertial plate or pulls the inertial plate and makes the inertial plate rotate following the rotation of the base lock.
An acceleration sensor is provided in a vicinity of the inertial plate. When a state of rapid deceleration of the vehicle arises, the acceleration sensor operates and restricts rotation of the inertial plate. At this time, due to the inertia at the time when the vehicle rapidly decelerates, the body of the vehicle occupant pulls the webbing belt such that the base lock is rotated in the pull-out direction via the spool, and relative rotation arises between the base lock and the inertial plate.
When such relative rotation arises, a lock member provided at the base lock is displaced, and meshes with internal ratchet teeth which are formed at the frame or the like of the webbing retractor. Rotation of the base lock in the pull-out direction is restricted, and accordingly, rotation of the spool in the pull-out direction is restricted. Due to the rotation being restricted, pulling-out of the webbing belt is impeded, and the body of the vehicle occupant can be reliably restrained and held by the webbing belt.
A force limiter mechanism is provided at the webbing retractor. When the body of the vehicle occupant pulls the webbing belt by a force of a predetermined magnitude or greater due to the inertia at the time when the vehicle rapidly decelerates, the force limiter mechanism permits a slight amount of pulling-out of the webbing belt while plastically deforming an energy absorbing member. The force limiter mechanism thereby absorbs and reduces the force (energy) which the webbing belt applies to the body of the vehicle occupant.
Specifically, the energy absorbing member, which is called a torsion shaft, is provided coaxially at the interior of the spool. The torsion shaft is integrally connected to the spool only at one axial direction end, and usually rotates integrally together with the spool.
The lock mechanism is connected integrally to the base lock at the other axial direction end side of the torsion shaft. The pinion of the pretensioner is provided coaxially with the torsion shaft, at the one axial direction end side of the torsion shaft. When the clutch mechanism is operated, the pinion and the torsion shaft are connected mechanically.
When the lock mechanism operates, the axial direction other end portion of the torsion shaft is locked, and rotation of the spool is restricted. In the state in which the lock mechanism is operated, when the vehicle occupant rotates the spool in the pull-out direction via the webbing belt, this torque is transmitted to the one axial direction end of the torsion shaft.
However, because the other axial direction end of the torsion shaft is locked by the lock mechanism, the one axial direction end of the torsion shaft rotates in the pull-out direction with respect to the other end. The torsion shaft thereby torsionally deforms. As described above, this torsional deformation of the torsion shaft absorbs and lessens the aforementioned energy.
At the time of this torsional deformation, the pinion also rotates in the pull-out direction. At the time when the vehicle rapidly decelerates, the pretensioner operates, and therefore, the pinion and the rack bar mesh together. Due to the sliding of the piston which accompanies the rise in internal pressure of the cylinder, the rack bar rotates the spool in the take-up direction. Therefore, when the pinion rotates in the pull-out direction as described above, the piston slides toward the floor portion against the internal pressure of the cylinder.
The internal pressure of the cylinder works to impede pull-out direction rotation of the one axial direction end of the torsion shaft via the rack bar and the pinion.
There is the problem that it is difficult to set the load needed to deform the torsion shaft, i.e., the so-called “force limiter load”.
Various structures have been proposed in order to overcome this problem (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-67300 and 2000-198413). In the structures disclosed in these publications, a gas vent hole or the like is formed in the piston or the cylinder.
In a structure in which a gas vent hole is formed in the piston, one end of the gas vent hole is open at the surface of the piston which surface is at the cylinder floor portion side. The other end of the gas vent hole is open at the side surface of the rack bar. Namely, in the state in which the rack bar is meshed with the pinion, when the pinion rotates in the pull-out direction and moves the rack bar toward the floor portion of the cylinder, accompanying the sliding of the piston, the gas remaining in the cylinder is made to pass from the gas vent hole toward the open end of the cylinder. In this way, the rise in the internal pressure of the cylinder accompanying the sliding of the piston toward the floor portion of the cylinder is prevented or lessened, and the effect on the force limiter load is reduced.
In a structure in which the gas vent hole is formed at the cylinder, the gas vent hole is formed further toward the floor portion side of the cylinder than the piston, and a valve is provided at the gas vent hole. When gas is supplied into the cylinder, the gas vent hole is closed by the valve. When the remaining gas is to be released when the piston is lowered, the valve is opened such that the gas vent hole is opened. In this way, the rise in the internal pressure of the cylinder accompanying the sliding of the piston toward the floor portion of the cylinder is prevented or lessened, and the effect on the force limiter load is reduced.
However, there are problems in that forming the gas vent hole in the piston and the rack bar, or forming the gas vent hole in the cylinder, is in and of itself difficult, and costs increase.